Continued device scaling of semiconductor devices leads to lithographic exposure of photoresist layers at, or near, the highest lithographic resolution that a lithographic exposure tool can provide. The minimum dimensions that are printed employing the highest lithographic resolution are commonly referred to as critical dimensions (CD's). Printing of the critical dimensions on a photoresist layer includes using an optimal focus distance between the optics system of the lithographic exposure tool and the photoresist layer as well as use of an optimal dose, i.e., an optimal amount of exposing (e.g., UV or X-ray) radiation that impinges onto the photoresist layer.
Variation of the focus distance across a wafer coated with a photoresist layer induces degradation of the printed image. To print the critical dimensions on the photoresist layer imposes a small focus budget on the lithographic exposure tool. In other words, small deviations from the optimal focus distance between the optics system of the lithographic exposure tool and the photoresist layer can render a developed pattern in the photoresist layer unusable.